1. Field of the Invention
The invention relates to component machining by electric discharge machining (EDM) or laser ablation machining of a work piece and subsequent removal of the ablation machining recast layer from the work piece by fluoride ion cleaning, so that the cleaned machined component has a desired shape and dimensions in accordance with component specifications. Exemplary embodiments of the invention relate to methods for machining turbine superalloy components, such as blades or vanes by EDM or laser ablation machining and subsequent removal of the ablation machining recast layer from the work piece by fluoride ion cleaning, so that the cleaned machined turbine component has a desired shape and dimensions in accordance with component specifications. The methods herein facilitate higher speed EDM or laser machining without fear of recast layer formation, and subsequent chemical removal of the recast layer, leaving an oxide-free component surface ready for further repair operations, such as brazing or welding. Embodiments of the present invention are suitable for machining cooling holes in superalloy turbine blades or vanes or formation of turbine seal slots.
2. Description of the Prior Art
Components, such as turbine blades and vanes, have complex, multi-dimensional geometric shapes are constructed of superalloy materials. Turbine component shape and superalloy machining complexities favor use of so-called “unconventional” ablation machining processes, such as laser machining or electrical discharge machining (EDM), compared to “traditional” cutting and grinding machining processes. Ablation machining can create a recast layer on the component machined surface comprising re-deposited carbonized molten metal particles, including oxides. Typically the recast layer is removed by conventional machining or chemical etching to conform the component's dimensions to the desired specifications. This removal adds additional steps, costs and time delays in the manufacturing process. For example, chemical etching processes leave undesirable oxide films on the workpiece surface and conventional machining leaves surface contaminants. The turbine component surfaces often require yet additional cleaning steps, such as fluoride ion cleaning (FIC) to prepare them for further manufacture or repair procedures, such as deposition of filler layers by welding or brazing processes.
In the past, ablation machining of components, such as nickel-based superalloy component used in turbine vanes and blades, has often focused on recast layer minimization or avoidance by lowering the component ablation rate, thereby slowing component production rate. Other past ablation machining recast layer avoidance solutions have been to ablate the component in a feedback loop, adjusting the ablation rate to minimize recast layer formation or sequential ablation rates that achieve cutting objectives, with subsequent recast layer removal as lower ablation rates. In the case of EDM processes electrode size and current application rates have been lowered to reduce recast layer formation, with or without the aid of feedback loops. In other EDM processes, sequential passes have been performed to cut most of the component surface at a relatively high speed, then lowering the current application and/or electrode size in final finishing passes, so that the final desired dimensional requirements are met without leaving any or at most a trace recast layer on the component surface that could be removed by minor conventional machining remediation operations.
Thus, a need exists in the art for a component ablation machining process (e.g., EDM) that incorporates an easily performed recast layer removal process, so that recast layer formation becomes less of a machining process concern. In this manner the ablation machining speed would be prioritized and optimized. Recast layer formation concerns would be addressed by knowledge that recast layer removal would be adequately addressed in the subsequent processing steps.